(1) Field of the Invention
The present invention relates to a butterfly valve for opening or closing a channel of fluid flowing at a high pressure in a facility for handling the fluid through a pipeline. More particularly, the present invention relates to a butterfly valve whose flexible sheet ring is not damaged by an excessive load due to the flow of a high-pressure high-speed fluid produced immediately before a valve disk is closed or immediately after the valve disk is opened and moreover, which makes it possible to control a very small amount of the fluid when the flow rate of the fluid is not more than 30% of the maximum flow rate (when the valve is fully opened), particularly not more than 10% of the maximum flow rate.
(2) Description of the Prior Art
For a general butterfly valve, a vertical shaft 2 is set to a short cylindrical valve body 1 in which fluid flows from the primary side (upstream side) toward the secondary side (downstream side) by slightly shifting the center of the shaft 2 from the center line C of the valve body 1 in the case of a double eccentric structure as shown in FIG. 11. A discoid valve disk 3 is secured to the shaft 2 to open or close a channel 4 of the valve body 1 by rotating the valve disk 2 about the shaft 2. To improve the airtightness of the valve disk 3, a sheet ring 5 is fitted into the inner periphery of the channel 4.
To open the butterfly valve by rotating the valve disk 3, one side 3a of the valve disk 3 is rotated toward the upstream side so as to go against the flow direction of the fluid and the other side 3b of the valve 3 is rotated toward the downstream side so as to follow the flow direction of the fluid as shown in FIG. 11(b). Then, in the case of the double eccentric structure, a gap is formed from the other side 3a of the valve disk 3 and thereby opening starts, the gap gradually expands toward the other side 3b of the valve disk 3, and finally the lateral end of the other side 3b separates. To close the butterfly valve, the above procedure is reversed.
In the process of opening or closing the butterfly valve, the speed of the fluid flowing through the channel 4 gets lowest when the valve disk 3 is fully opened (approx. 90.degree.) and gets highest immediately after opening starts or immediately before closing starts as shown in FIG. 11(b).
In the case of a conventional butterfly valve, the change of flow rate is larger than the change of valve opening rate over the entire valve opening rate like the characteristic curves shown by an alternate long and short dash line in FIG. 7 and moreover, the flow rate comes to 0 at an opening rate of 5 to 10%. Therefore, there are problems that fine control of a flow rate of 10% or less is difficult and a flow rate of 5% or less can hardly be controlled.
This is because the flow velocity of the fluid extremely increases when the valve opening rate is 20% or less and the sheet ring 5 is deformed due to the load when the fluid collides with the ring 5 by jetting from a narrowed portion to change the effective passing area of the channel 4. The load applied to the sheet ring 5 is arithmetically almost proportional to (amount of fluid hitting the sheet ring 5).times.(flow velocity).sup.2. When the valve opening rate increases to a certain extent, the flow velocity decreases and therefore, deformation of the sheet ring 5 is very small. Thus, even if the effective passing area of a channel is changed due to the deformation of the sheet ring 5, the change can be ignored. However, when the valve opening rate extremely decreases, the change of the effective passing area cannot be ignored. Moreover, when the valve opening rate deceases, the flow velocity of the fluid increases to accelerate the deformation of the sheet ring 5. Specifically, the load applied to the sheet ring 5 is generally maximized when the valve opening rate is approximately 15%.
However, when the closing rate of the valve disk 3 exceeds a limit, the value of (amount of fluid hitting the sheet ring 5) starts decreasing, the load applied to the sheet ring 5 does not increase even if (flow velocity) increases. Moreover, the load decreases as the value of (amount of fluid hitting the sheet ring 5) decreases. Then, the deformation of the sheet ring 5 decreases, the channel narrows, the effective passing area acceleratedly decreases, and the valve disk 3 closes. Therefore, the flow rate suddenly decreases from a certain level when the valve opening rate is 10% or less and moreover, the critical point changes depending on the then state (closing speed or temperature of the valve disk 3, specific gravity or viscosity of fluid, or presence or absence of vibration). Therefore, it is impossible to control a flow rate in accordance with a valve opening rate.
Moreover, as described above, the flow velocity of the conventional butterfly valve is maximized at a valve opening rate of approximately 15% immediately after the lateral end of the other side 3a of the valve disk 3 opens or immediately before it closes and the load applied to the sheet ring 5 is maximized in this case. Furthermore, the fluid collides with the sheet ring 5 located at the downstream side of the opening of the valve disk 3a so as to tear up the front end of the ring 5 in the form of a jet flow or turbulent flow. Therefore, there is a problem that the lateral end of the sheet ring 5 located at the other side 3a of the valve disk 3 is easily damaged.
It is an object of the present invention to provide a butterfly valve firstly having a characteristic capable of finely control a flow rate of 10% or less of the total flow rate and secondly almost preventing a sheet ring from damaging.